In two-way radio systems in which a plurality of radio channels can be used to communicate between transmitter stations and mobile transmitter/receivers, it is common for the geographic areas covered by some of the channels to overlap to some extent. Thus, it frequently occurs that these overlapping channels can be separated from the remainder of the radio channels into groups or blocks of geographically common coverage. Furthermore, it is often the case that the geographic area covered by one group of channels may be located so that there is no overlap with the geographic coverage of another group of channels.
Mobile transmitter/receiver devices, such as vehicular mounted units (mobile units) or handheld units (portable units), that can move freely throughout wide-ranging areas must be able to detect messages on all radio channels used in the areas in which it moves. In prior art mobile radio devices, all of the channels that the device might potentially use in any of its operational area were scanned in order to be certain that all messages directed to the unit had been received. A problem with these prior art radio devices is that a significant amount of time is required to scan and monitor each of the channels that the device would be programmed to receive. The time required for scanning every channel created the risk that some messages sent to the radio device on a particular channel might not be detected because the device was busy scanning other channels for messages at the time of reception. This problem is exacerbated with radio systems consisting of a large number of channels. For example, trunking systems are radio systems containing multiple radio channels that are automatically shared by all of the mobile or portable radio assigned to the system. The greater the number of channels required to be monitored by a particular receiving device, the greater likelihood that a message signal directed to the receiving device would not be detected.
In view of this problem, there exists a need for a method and apparatus for efficiently monitoring a plurality of radio channels for message signals to reduce the amount of time spent scanning radio channels, thereby increasing the likelihood of receiving a message directed to the receiving device.